Leveraged positions in the stock market jumped 18% in one month and topped $1 trillion for the first time. Increasing use of debt to invest can tell us a lot about investors' attitudes. And while this has always been the case,

Buffett owned UnitedHealth shares before selling his stake in 2010, though his Berkshire Hathaway recently acquired shares.

Given Buffett's concern over the stock market, it makes sense for investors to buy the most conservative stocks in the Berkshire Hathaway portfolio.

Warren Buffett is telling investors something -- not through what he has said, but through his actions with the balance sheet of his investment conglomerate Berkshire Hathaway ( BRK.A -0.40%) ( BRK.B 0.83%). These actions should be taken as a warning sign with the stock market reaching elevated valuation levels in recent months.

Beaten-down UNH is set to challenge its 50-day moving average after a major vote of confidence from the Oracle of Omaha.

Intel rallies as U.S. discusses taking stake in chip maker

The cyclically adjusted price-to-earnings (CAPE) ratio is a valuation metric typically used to gauge whether entire stock market indexes are overvalued or undervalued. It was developed by Nobel-winning economist Robert Shiller during the dot-com bubble, so it is sometimes referred to as the Shiller P/E ratio.